The invention moreover concerns a method for operating the air conditioning system.
The air conditioning systems found in the prior art with the functions of heating, cooling, and dehumidification serve for the conditioning of the air being introduced into the passenger compartment of motor vehicles with electric and hybrid drive system, as well as internal combustion engine system.
Air conditioning systems of this kind with a coolant-air heat exchanger or heating heat exchanger, which draw their heating power from the coolant circuit of an efficient internal combustion engine or a hybrid drive system, on the one hand no longer achieve the required temperature level for a comfortable heating of the passenger compartment when outside temperatures are low, such as below −10° C. For these vehicles, the use of auxiliary heating concepts is necessary. On the other hand, heat is removed from the coolant, so that the internal combustion engine is operated for a lengthy time at low temperatures, which has negative impact on the exhaust emission and the fuel consumption. Due to the intermittent operation of the internal combustion engine in hybrid vehicles, it is no longer possible to achieve sufficiently high coolant temperature during a lengthy trip. Consequently, the start and stop operation of the internal combustion engine is disabled when outside temperatures are low. The internal combustion engine is not switched off.
Furthermore, there is a trend to complete electrification of the drive system, as in the case of vehicles driven by batteries or fuel cells. This eliminates the waste heat of the internal combustion engine as a possible heat source for warming the air.
Moreover, the quantity of energy which can be stored in the battery of the vehicle is less than the amount of energy which can be stored in the form of liquid fuel within the fuel tank. Thus, the power required for the air conditioning of the passenger compartment of an electrically operated vehicle also has a substantial influence on the range of the vehicle.
In European Pat. Appl. Pub. No. EP 2 629 040 A2 a compact heat pump system is disclosed for a motor vehicle with a heat exchanger unit, having a condenser, an integrated collector, and an undercooling heat exchanger of a refrigerant circuit. The heat exchanger unit connects the refrigerant circuit to a high temperature coolant circuit. The refrigerant circuit furthermore has a heat exchanger operating as an evaporator, constituting a connection to a low temperature coolant circuit. The refrigerant circuit consequently connects the high temperature coolant circuit to the low temperature coolant circuit. The low temperature coolant circuit is designed in particular to remove heat from components of a drive unit, the heat being transferred from the coolant to the refrigerant in the evaporator. The refrigerant surrenders the [missing object—heat?] taken up from the low temperature coolant in the condenser or undercooling heat exchanger once more to the high temperature coolant. The high temperature coolant transfers the heat taken up either to an air mass flow being supplied to the passenger compartment or to the outside air.
Consequently, the heat being transferred is surrendered by the low temperature coolant entirely to the refrigerant and by the refrigerant entirely to the high temperature coolant.
In the known heat pump systems, the desuperheating heat and the heat of condensation as well as the undercooling heat of the refrigerant is transferred entirely to the high temperature coolant. The heat can then be surrendered upon flowing through a coolant-air heat exchanger from the coolant to the air being supplied to the passenger compartment.
The heat could also be taken away to the surroundings or be utilized for heating the battery, for example.
Each time the heat is transferred indirectly by one or more coolant circuits. In the principle of indirect heat transfer, the efficiency of the overall system depends critically on the return temperature of the coolant at the entrance to the coolant-cooled condenser/gas cooler. When the coolant-cooled condenser/gas cooler is designed as a counterflow heat exchanger, the refrigerant can be cooled at most down to the entry temperature of the coolant in the condenser/gas cooler. Consequently, as the return temperature of the coolant becomes higher, the exit temperature of the refrigerant from the condenser/gas cooler also becomes larger.
Especially when an air conditioning system is operated in the heat pump mode at very low temperatures of the outside air in the range of −15° C. to −20° C., it is desirable to have return temperatures of the coolant in the range of around 55° C. to 60° C., in order to condition the passenger compartment in the shortest possible time to acceptable temperatures. In such an operating mode, the refrigerant upon leaving the condenser/gas cooler has a maximum temperature of over 55° C.
But in order to operate the air conditioning system in a very efficient manner, the refrigerant must be cooled to a temperature far below a return temperature of the coolant of 55° C.
In U.S. Pat. No. 8,156,754 B2 one finds an air conditioning system of a motor vehicle with a refrigerant circuit which is connected via a coolant-cooled heat exchanger to a high temperature coolant circuit. Upon flowing through the coolant-cooled heat exchanger, the hot gaseous refrigerant emerging from the compressor of the refrigerant circuit is desuperheated before the refrigerant leaving the coolant-cooled heat exchanger flows into a condenser/gas cooler. Upon flowing through the condenser/gas cooler additional heat is surrendered by the refrigerant to the surrounding air. The refrigerant is liquefied.
The heat surrendered in the coolant-cooled heat exchanger to the coolant is on the one hand dependent on the efficiency of the heat exchanger and on the other hand on the temperature level of the coolant. The possibility exists of dividing the desuperheating heat and the heat of condensation of the refrigerant as so-called heat loads between two heat exchangers. In this way, on the one hand, one can achieved a higher performance of the air conditioning system and on the other hand the condenser/gas cooler can be designed with smaller dimensions.
The heat transferred by the coolant-cooled heat exchanger from the refrigerant to the coolant is given off to the surrounding air in a coolant-air heat exchanger and thus diverted out of the system. The heat surrendered to the surrounding air is consequently not usable for the heating of the passenger compartment. Furthermore, the air conditioning system according to U.S. Pat. No. 8,156,754 B2 is not designed with a heat pump functionality, by which the passenger compartment can be heated in event of little or no available engine heat, especially when outdoor temperatures are low. Both the condenser/gas cooler and the evaporator will be supplied with outdoor air.
The systems known in the prior art are each designed with a refrigerant circuit which can only operate in one operating mode. The refrigerant circuits have no refrigerant-air heat exchanger for heating an air mass flow being supplied to the passenger compartment. Furthermore, the refrigerant-air heat exchangers are constantly supplied with surrounding air, which is diverted to the outside after being cooled down in the passenger compartment or after being heated. Thus, the heat carried away by the refrigerant cannot be used directly for the air mass flow being supplied to the passenger compartment.
The problem which the invention proposes to solve is to provide an air conditioning system for a motor vehicle with a refrigerant circuit with heating functionality. The system should furthermore be designed for the combined operation in refrigerator and heat pump mode and for afterheating mode for the heating, cooling, and dehumidification of the air being conditioned in the passenger compartment. It should also be possible to operate in environments with heat sources of low capacity, especially with slight waste heat of the drive system of the motor vehicle, such as in the case of energy-efficient internal combustion engine drive units and hybrid drive units composed of an internal combustion engine and an electric motor, or in electrically propelled motor vehicles, fulfilling all requirements for a comfortable climate in the passenger compartment. The comfort should be constantly maintained regardless of the operating mode, in particular, a loss of thermal power should be prevented. The refrigerant circuit should be easily composed of a minimal number of components and be able to operate in different operating modes. The system should also be usable in already existing motor vehicle architectures.